When maintenance staff working on the ventilation system had completed their work , they decided to switch the ventilation back on . To maintain the depression in air pressure in the building , the practice was to switch on the extract first and check this was functioning correctly before switching on the plenum .
With the extract running but no plenum to balance the air , the reduction in air pressure was so great that it sucked the tent into the building , bending the frame and leaving the contaminated drain exposed to the external atmosphere .
Fortunately , plenty of people were on site to respond and cover the contaminated area . No contamination was released to the environment , and the tent was safely repaired and reinstalled .
Lesson learned – make sure that when a Permit to Work is drawn up , everyone likely to be affected is consulted , and the impact of any other work is considered .